The present invention relates to medical imaging.
Intravascular ultrasound (IVUS) imaging provides medical professionals with real-time, cross-sectional, high-resolution images of the arterial lumen and vessel wall. IVUS imaging permits visualization of lesion morphology and accurate measurements of arterial cross-sectional dimensions in patients. This has led to many important clinical applications including quantitative assessment of the severity of restenosis or the progression of atherosclerosis, the selection and guidance of catheter-based therapeutic procedures, and evaluation of the outcome of intravascular intervention. For example, to assess the level of plaque build-up within an artery, the lumen's border and the artery's border can be detected. The level of plaque is typically the difference between the two borders.
A conventional technique for generating a cross-sectional intravascular ultrasound (IVUS) image of a vessel involves sweeping an ultrasound beam sequentially in a 360-degree scan angle. A single element transducer at the end of a catheter can be rotated inside the vessel. Either the single element transducer can be attached to a flexible drive shaft or a rotating mirror can be used; in either case, the ultrasound beam is directed to substantially all angular positions within the vessel. Alternatively, a large number of small transducer elements can be mounted cylindrically at the circumference of the catheter tip, and the ultrasound beam steered electronically to form a cross-sectional scan.
The interaction of the ultrasound beam with tissue or blood yields an echo signal that is detected by the transducer. Based upon the biological medium that the echo signal interacts with, the echo signal can experience attenuation, reflection/refraction, and/or scattering. When an ultrasound wave travels across the boundary between two types of media, part of the wave is reflected at the interface, while the rest of the wave propagates through the second medium. The ratio between the reflected sound intensity and the intensity that continues through to the second medium is related to the difference in acoustic impedance between the mediums. An IVUS system includes conversion circuitry to convert the echo signals described above into electronic signals capable of being displayed as an ultrasound image, e.g., in a standard video format.